As RF systems' operating frequencies continue to rise, system reliability becomes increasingly reliant on hermetic or near hermetic packaging materials. Higher frequencies lead to several design challenges such as greater circuit sensitivity to tolerances (due to their smaller size), more difficult design of RF interconnects and package feedthroughs (due to high frequency parasitics), and greater sensitivity to packaging material characteristics (due to higher inherent material losses and absorbed water/oxygen contributions at higher frequencies). Low material expansion coefficients for packaging materials that are matched to the metals and/or semiconductors they are mated with are increasingly important as well since material expansion is also related to water absorption characteristics. Finally, moisture permeability should be minimized to maintain a stable dielectric constant and loss tangent.
The best packaging materials in terms of hermeticity are metals, ceramics, and glass. However, these materials often give way to cheaper polymer packages such as injection molded plastics or glob top epoxies when:
Packaging systems and methods of manufacture are provided. In this regard, an embodiment of such a system comprises a first layer of liquid crystal polymer (LCP), an electronic component supported by the first layer, and a second layer of LCP. The first layer and the second layer encase the first electronic component.
Another embodiment of such a system comprises: a first layer of liquid crystal polymer (LCP), the first layer being substantially planar; a first electronic component supported by and physically contacting the first layer; and a second layer of LCP having a cavity formed therein. The cavity is sized and shaped to receive at least a portion of the electronic component therein. The first layer and the second layer are arranged in an overlying relationship with respect to each other and fixed in position with respect to each other such that the first electronic component is near-hermetically sealed within the cavity, with the first electronic components being encased within the cavity by LCP of the first layer and the second layer.
An embodiment of a method comprises: providing a first layer and a second layer of liquid crystal polymer (LCP); supporting a first electronic component with the first layer; and encasing the first electronic component with the first layer and the second layer.
In the drawings, like reference numerals indicate corresponding components. Additionally, the drawings are not necessarily to scale.
As will be described in detail here, packaging systems and methods of manufacture are provided. In particular, such systems and methods involve the use of liquid crystal polymer (LCP). LCP exhibits a low dielectric constant and low loss tangent in tandem with low water absorption coefficient and low cost. Additionally, the coefficient of thermal expansion (CTE) of LCP can be adjusted, such as through thermal treatments, for example.
Solid state devices, such as pin diodes, have been packaged in LCP. In addition, several companies have recently developed injection molded LCP packaging caps, which can be used to seal individual components with epoxy or laser sealing. However, these packages can be bulky which may limit the packaging integration density. In addition, these rigid packaging caps (LCP becomes rigid when it has sufficient thickness) can take away one of the LCP substrates very unique characteristics—flexibility.
In this regard, an embodiment of a packaging system is depicted schematically in
A second layer 16 of LCP is then provided to encase the component 14. In some embodiments, as in this case, the first and second layers form a near-hermetic enclosure about the component 14. As used herein, the term “near-hermetic” means offering a hermetic seal over a limited, but substantial period of time. LCP's hermeticity has often been compared to that of glass which has a very low, but measurable permeability to moisture and gas. This corresponds to a package that could claim hermeticity for a number of years and satisfy the lifetime requirements for numerous applications. The barrier thickness would determine the time before some level of moisture and gas permeation pass through the barrier. The fluoropolymers like Teflon are the only polymers that compare similarly in terms of water permeability, but Teflon is worse than LCP in terms of gas permeability. In addition, LCP's multilayer lamination capability from the high and low melting temperature types allows for a unique capability of a sealed homogeneous LCP structure composed of the “near-hermetic” properties. Oxygen permeability rates for LCP are on the order of 0.02 [(cm3*mm)/(m2*day*atm)] and water permeability is on the order of 0.009 [(g*mm)/(m2*day)].
Notably, each of the layers is a thin-film LCP layer, thus, if desired, the material flexibility of the system may be retained by providing an appropriate overall thickness of the layers. If rigidity is desired, if taller cavities are required, or if lower package permeability is a goal, more thin-film layers may be laminated together to achieve the desired package characteristics and geometry.
In the embodiment of
An embodiment of a method for manufacturing a packaging system, such as that of
Since LCP has a low dielectric constant near 3.16 (close to free space εr=1), impedance mismatches are minimal when an LCP superstrate layer is added on top of a standard transmission line. In addition, if cavities are machined in the superstrate layer, the cavities do not create large impedance mismatches at the cavity interface. Thus, LCP's low dielectric constant enables package cavities of arbitrary size to be integrated in a superstrate packaging layer to accommodate chips, MEMS, or other devices without concern for parasitic packaging effects.
The RF characteristics of transmission lines with superstrate layers and various packaging cavities were investigated. In this regard,
The impedance difference between these simulations is 4Ω (see corresponding impedance adjacent each cross section). An impedance difference of only 4Ω between a transmission line with a superstrate layer versus those with a cavity (
To determine the effects of varying the cavity dimensions, the impedance of each cross section of the embodiments of
To demonstrate this capability, a 4 mil non-metallized LCP superstrate layer with depth-controlled laser micromachined cavities was constructed as a package. This technique is demonstrated by creating packages for air-bridge RF MEMS switches. The switch membranes are only about 3 μm above the base substrate which allows a cavity with plenty of clearance to be laser drilled in the LCP superstrate layer. A cavity depth of 2 mils (˜51 μm), half of the superstrate thickness, was chosen for the MEMS package cavities.
This technique could be extended to include additional layers as necessary. To accommodate devices that require more vertical clearance, multiple LCP layers could have holes or cavities formed therein, such as by drilling, and the layers stacked together. The packages can be sealed with thermo-compression, ultrasonic, or laser bonding, for example.
Several advantages of the above-mentioned technique are: the flexibility of the substrate may be maintained for applications such as conformal antennas, the package is light weight, and the LCP packaging layer is a standard inexpensive microwave substrate which can be made into any system-level package configuration. Two primary applications are large-scale antenna arrays with packaged ICs and/or switches inside of a multi-layer antenna substrate, or vertically integrated LCP-based RF modules where switches and/or active devices may be bonded inside of a multi-layer LCP construction.
A CO2 engraving laser with a 10 μm wavelength was used to form holes in the LCP superstrate layer (see
Due to surface irregularities caused during formation of the holes by the CO2 engraving laser, a cleaning process was conducted. In particular, the LCP superstrate was cleaned by a plasma cleaning process to remove the irregularities as shown in
Next, an excimer laser was used to micromachine depth-controlled cavities 56 in the desired locations (see
The laser power and the number of pulses were tuned to provide the desired ablation depth into the LCP superstrate. We arbitrarily chose to make cavity depths half of the substrate thickness (2 mil deep cavities). Shallower or deeper cavities are possible by varying the laser power and the number of pulses. A custom brass aperture with a rectangular hole was used to shape the beam to the desired cavity shape and size. This aperture size of 12 mm×5 mm was demagnified five times to create a cavity 2.4 mm wide×1 mm long. After machining the cavities, the depth was checked with a microscope connected to a digital z-axis focus readout with accuracy to the nearest tenth of a micron. The depth across the bottom of the cavities was not completely uniform due to some small burn marks on the laser optics, but it was within ±5 microns of the desired depth across the entire cavity.
Due to surface irregularities caused during formation of the cavities by the excimer laser, a cleaning process was conducted. In particular, the LCP superstrate was cleaned by a plasma cleaning process to remove the irregularities as shown in
The completed package layers were made such that the alignment holes 52 corresponded to the same location as those on the through-reflect-line (TRL) calibration lines and also on the MEMS switch samples. Note in
As shown in
Because MEMS switches are by nature fragile, an iterative measurement procedure was undertaken. First, the switches were measured in air to provide a base measurement case. The second and third measurements were done with the package layer aligned and held into contact with the base substrate. The first packaging iteration was done by gently holding the package layer down over the MEMS substrate with tape. When the switches continued to operate with the package layer in place, this ensured that the alignment of the package cavities was successful. Finally, the top metal plate was placed over the alignment pins and a fifteen pound weight was balanced on top of the samples (see
In an actual bonding process for this particular package, a hole could be cut in a low-melt 1 mil LCP bond ply to form a 1 mil (25 μm) cavity, which would require a hole to be drilled in the bond ply rather than a depth-controlled cavity in a core layer. (see
Specifically,
The S-parameters of the packaged switch and the non-packaged switch are nearly identical in both the up and down states. For example, the variation between the three measurement cases for S21 in the UP state only varies by an average of 0.032 dB across the entire measurement.
To show the effects of the packaging layer and cavity on a simple transmission line, the switch membrane was physically removed and the circuit re-measured. The results of the bare transmission line with and without the packaging layer are shown in
Fabrication of an embodiment of an RF MEMS switch such as mentioned above will now be described in greater detail. In particular, clamped-clamped (air-bridge-type) and clamped-free (cantilever-type) coplanar waveguide (CPW) switches with a membrane size of 100 μm×200 μm and various hinge geometries (solid and meander shaped) were fabricated on LCP substrates using a four mask low-temperature process that reduces the surface roughness and assures good switch performance.
An embodiment of the four mask process is shown in
Measurements of the air-bridge type switch were taken using an Agilent 8510 network analyzer. A TRL calibration was performed to de-embed the coplanar line and transition losses. Measured results for the nitride switches with silicon substrate and LCP are shown in
For the same switch on silicon substrate, when the switch is activated, the isolation is around 17 dB at 20 GHz and CON=2 pF, while the return loss is around 0.4 dB at 20 GHz. When the switch is in the UP position, the insertion loss is around 0.8 dB at 20 GHz and COFF=25 fF; the return loss is 10 dB at 20 GHz. The deteriorated return loss of the switch on silicon is due to the thinner sacrificial layer that increases the capacitance, while the different CON between the two types of switches with different substrate is because the thickness of silicon nitride is a little different.
The measured air-bridge switches with an LCP substrate gave better insertion loss in the up state than that of the switches on the silicon substrate. The switches on LCP also gave better isolation in the down state. Additionally, due to LCP's low dielectric constant, the air/dielectric discontinuities in the packaging structures are insignificant. Thus, the package cavities can be designed almost arbitrarily without concern for their effect on RF performance. The foregoing description has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise forms disclosed. Modifications or variations are possible in light of the above teachings. By way of example, the layer count, layer thicknesses, etc. can be varied and the supported devices can be placed in various locations on and/or between the layers as desired. In particular, one package technique could be used to put packages on any single layer (likely implemented as a hole in the bond ply surrounded by two solid core layers), or by having holes in multiple layers and have the layers stacked to create taller cavities.
By way of further example, although described with reference to packaging of MEMS switches, the technique aforementioned techniques could be used broadly for integrated circuit (IC) packaging, or generally for any active or passive electronic component to be packaged in a multilayer LCP topology. Due to LCP's bonding temperature around 285° C., it is possible that ICs could be packaged inside the LCP package cavity without damaging the IC.
This application is based on and claims priority to U.S. Provisional Patent Application Ser. No. 60/657,814, filed on Mar. 2, 2005, which is incorporated by reference herein.
The U.S. government may (or does) have a paid-up license in this invention(s) and the right in limited circumstances to require the patent owner to license others on reasonable terms as provided for by the terms of NASA contract NCC3-1057.
Filing Document | Filing Date | Country | Kind | 371c Date |
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PCT/US05/42737 | 11/23/2005 | WO | 00 | 8/31/2007 |
Number | Date | Country | |
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60657814 | Mar 2005 | US |